[13-Jul-2007] Today it is exactly three years ago since PHP 5 has been released. In those three ears it has seen many improvements over PHP 4. PHP 5 is fast, stable & production-ready and as PHP 6 is on the way, PHP 4 will be discontinued.
The PHP development team hereby announces that support for PHP 4 will continue until the end of this year only. After 2007-12-31 there will be no more releases of PHP 4.4. We will continue to make critical security fixes available on a case-by-case basis until 2008-08-08. Please use the rest of this year to make your application suitable to run on PHP 5.
For documentation on migration for PHP 4 to PHP 5, we would like to point you to our migration guide. There is additional information available in the PHP 5.0 to PHP 5.1 and PHP 5.1 to PHP 5.2 migration guides as well.

Taken from http://www.php.net/